..And you'll want 50fps for PAL games, or 60fps for NTSC games. Everything else isn't really enjoyable much.
So yeah, depending on the game you need a core2duo 2.4Ghz (easy to run game) up to core2duo 4.5Ghz (the tough ones ).

Edit: And yeah, I know you cannot even buy the 4.5Ghz one (only heavy overclocking gets you there). It's the way it is, sorry